1. Field of the Invention
This invention relates to the manufacture of an integrated circuit and, more particularly, to the formation of an n-channel and/or p-channel asymmetrical transistor having a gate oxide incorporated with barrier atoms to enhance transistor performance.
2. Description of the Related Art
Fabrication of a MOSFET device is well known. Generally speaking, MOSFETs are manufactured by placing an undoped polycrystalline (xe2x80x9cpolysiliconxe2x80x9d) material over a relatively thin gate oxide. The polysilicon material and gate oxide is then patterned to form a gate conductor formed laterally between exposed regions of single crystalline silicon substrate. The gate conductor and exposed substrate regions are then implanted with an impurity dopant material. If the impurity dopant material used for forming the junction areas within the exposed substrate is n-type, then the resulting MOSFET is an NMOSFET (xe2x80x9cn-channelxe2x80x9d) transistor device. Conversely, if the dopant material is p-type, then the resulting MOSFET is a PMOSFET (xe2x80x9cp-channelxe2x80x9d) transistor device.
The gate conductor and adjacent implant areas (herein xe2x80x9cjunctionsxe2x80x9d) are formed using well known photolithography and ion implant techniques. Gate conductors and implant regions arise in openings formed through a thick dielectric layer of what is commonly referred to as field oxide. Those openings and the transistors formed therein are termed active regions. The active regions are therefore regions between field oxide regions. Metal interconnect is routed over the field oxide to couple with the polysilicon gate conductor as well as with the junction to complete the formation of an integrated circuit.
Integrated circuits utilize either n-channel devices exclusively, p-channel devices exclusively, or a combination of both on a single monolithic substrate. While both types of devices can be formed, the devices are distinguishable based on the dopant species used. The method by which n-type dopant is used to form an n-channel device and p-type dopant is used to form a p-channel device entails unique problems associated with each device. As layout densities increase, the problems are exacerbated. Device failure can occur unless adjustments are made to processing parameters and processing steps. N-channel processing must, in most instances, be dissimilar from p-channel processing due to the unique problems of n-channel transistors relative to each type of device.
N-channel devices are particularly sensitive to so-called short-channel effects (xe2x80x9cSCExe2x80x9d). The distance between a source-side junction and a drain-side junction is often referred to as the physical channel length. However, after implantation and subsequent diffusion of the junctions, the actual distance between junctions becomes less than the physical channel length and is often referred to as the effective channel length (xe2x80x9cLeffxe2x80x9d). In VLSI designs, as the physical channel becomes small, so too must the Leff. SCE becomes a predominant problem whenever Leff drops below, e.g., 2.0 xcexcm.
Generally speaking, SCE impacts device operation by, inter alia, reducing device threshold voltages and increasing sub-threshold currents. As Leff becomes quite small, the depletion regions associated with the source and drain areas within the junctions may extend toward one another and substantially occupy the channel area. Hence, some of the channel will be partially depleted without any influence of gate voltage. As a result, less gate charge is required to invert the channel of a transistor having a short Leff. Somewhat related to threshold voltage lowering is the concept of sub-threshold current flow. Even at times when the gate voltage is below the threshold amount, current between the source and drain nonetheless exist for transistors having a relatively short Leff.
Two of the primary causes of increased sub-threshold current are: (i) punch through and (ii) drain-induced barrier lowering (xe2x80x9cDIBLxe2x80x9d). Punch through results from the widening of the drain depletion region when a reverse-bias voltage is applied across the drain-well diode. The electric field of the drain may eventually penetrate to the source area, thereby reducing the potential energy barrier of the source-to-body junction. Punch through current is therefore associated within the substrate bulk material, well below the substrate surface. Contrary to punch through current, DIBL-induced current occurs mostly at the substrate surface. Application of a drain voltage can cause the surface potential to be lowered, resulting in a lowered potential energy barrier at the surface and causing the sub-threshold current in the channel near the silicon-silicon dioxide interface to be increased. One method in which to control SCE is to increase the dopant concentration within the body of the device. Unfortunately, increasing dopant within the body deleteriously increases potential gradients in the ensuing device.
Increasing the potential gradients produces an additional problems known as hot-carrier effect/injection (xe2x80x9cHCIxe2x80x9d). HCI is a phenomena by which the kinetic energy of the carriers (holes or electrons) is increased as they are accelerated through large potential gradients and subsequently become trapped within the gate oxide. The greatest potential gradient, often referred to as the maximum electric field (xe2x80x9cEmxe2x80x9d) occurs near the drain during saturated operation. More specifically, the electric field is predominant at the lateral junction of the drain adjacent the channel.
Using the n-channel example, the electric field at the drain causes channel electrons to gain kinetic energy. Electron-electron scattering randomizes the kinetic energy and the electrons become xe2x80x9chotxe2x80x9d. Some of these hot electrons have enough energy to create electron-hole pairs through impact ionization of the silicon atoms. Electrons generated by impact ionization join the flow of channel electrons, while the holes flow into the bulk to produce a substrate current in the device. The substrate current is the first indication of the creation of hot carriers in a device. For p-channel devices, the fundamentals of the process are essentially the same except that the role of holes and electrons are reversed.
HCI occurs when some of the hot carriers are injected into the gate oxide near the drain-side junction, where they induce damage and become trapped. Traps within the gate oxide generally become electron traps, even if they are initially filled with holes. As a result, there is a negative charge density in the gate oxide. The trapped charge accumulates with time, resulting in positive threshold shifts in both n-channel and p-channel devices. It is known that since hot electrons are more mobile than hot holes, HCI causes a greater threshold skew in n-channel devices than p-channel devices. Nonetheless, a p-channel device will undergo negative threshold skew if its Leff is less than, e.g., 0.8 xcexcm.
Unless modifications are made to the transistor structure, problems of sub-threshold current and threshold shift resulting from SCE and HCI will remain. To overcome these problems, alternative drain structures such as double-diffused drains (DDDs) and lightly doped drains (LDDs) must be used. The purpose of both types of structures is the same: to absorb some of the potential into the drain and thus reduce Em. The popularity of DDD structures has given way to LDD structures since DDD causes unacceptably deep junctions and deleterious junction capacitance.
A conventional LDD structure is one whereby a light concentration of dopant is self-aligned to the gate conductor followed by a heavier dopant self-aligned to the gate conductor on which two sidewall spacers have been formed. The purpose of the first implant dose is to produce a lightly doped section within the junction at the gate edge near the channel. The second implant dose is spaced from the channel a distance dictated by the thickness of the sidewall spacer. The second implant dose is the source/drain implant placed within the junction laterally outside the LDD area, also within the junction. Resulting from the first and second implants, a dopant gradient (i.e., xe2x80x9cgraded junctionxe2x80x9d) occurs at the interface between the source and channel as well as between the drain and channel.
A properly defined LDD structure must be one which minimizes HCI but not at the expense of excessive source/drain resistance. The addition of an LDD implant adjacent the channel unfortunately adds capacitance and resistance to the source/drain path. This added resistance, generally known as parasitic resistance, can have many deleterious effects. First, parasitic resistance can decrease the saturation current (i.e., current above threshold). Second, parasitic capacitance can decrease the overall speed of the transistor.
The deleterious effects of decreased saturation current and transistor speed is best explained in reference to a transistor having a source and drain parasitic resistance. This resistance is compounded by the presence of the conventional source and drain LDDs. Using a n-channel example, the drain resistance RD causes the gate edge near the drain to xe2x80x9cseexe2x80x9d a voltage, e.g., less than VDD, to which the drain is typically connected. Similarly, the source resistance RS causes the gate edge near the source to see some voltage, e.g., more than ground. As far as the transistor is concerned, its drive current along the source-drain path depends mostly on the voltage applied between the gate and source, i.e., VGS. If VGS exceeds the threshold amount, the transistor will go into saturation according to the following relation:
IDSAT=K/2*(VGSxe2x88x92VT)2
,where IDSAT is saturation current, K is a value derived as a function of the process parameters used in producing the transistor, and VT is the threshold voltage. Reducing or eliminating RS would therefore draw the source-coupled voltage closer to ground, and thereby increase the effective VGS. From the above equation, it can be seen that increasing VGS directly increases IDSAT. While it would seem beneficial to decrease RD as well, RD is nonetheless needed to maintain HCI control. Accordingly, a substantial LDD area is required in the drain area.
Proper LDD design must take into account the need for minimizing parasitic resistance RS at the source side while at the same time attenuating Em at the drain side of the channel. Further, proper LDD design requires that the injection position associated with the maximum electric field Em be located under the gate conductor edge, preferably well below the silicon surface. It is therefore desirable to derive an LDD design which can achieve the aforesaid benefits while still properly placing and diffusing Em. This mandates that the channel-side lateral edge of the LDD area be well below the edge of the gate. Regardless of the LDD structure chosen, the ensuing transistor must be one which is not prone to excessive sub-threshold currents, even when the Leff is less than, e.g., 2.0 xcexcm.
A properly designed LDD-embodied transistor which overcomes the above problems must therefore be applicable to either an n-channel transistor or a p-channel transistor. That transistor must be one which is readily fabricated within existing process technologies. In accordance with many modem fabrication techniques, it would be desirable that the improved transistor be formed having a net impurity concentration within the polysilicon gate of the same type as the junction regions (i.e., LDD implant and/or source/drain regions).
The problems outlined above are in large part solved by an improved transistor configuration. The transistor can be either a p-channel or n-channel transistor. The transistor hereof is classified as an asymmetrical transistor in that the LDD implant is purposefully formed exclusively in the drain-side junction between the channel and the drain region. Thus, LDD area is eliminated from the source-side of the transistor.
A net LDD focused primarily at the drain side maintains parasitic resistance of the drain LDD (i.e., RD) but reduces if not eliminates LDD-induced parasitic resistance RS associated with the source-side LDD implant. Thus, the drain-engineered structure hereof serves to attenuate the maximum electric field Em in the critical drain area while reducing parasitic resistance RS in the source area. The drain-side LDD region is bounded by a junction which exists below the gate edge and below the silicon surface. The LDD area, however, is attributed solely or primarily to the critical area near the drain. Shifting the electric field, Em, occurs only in the region where shifting is necessary, i.e., only in the drain-side of the channel.
The p- or n-channel transistor formed as a result of the present LDD design receives the benefit of reduced HCI but not at the expense of performance (i.e., switching speed or saturation current). Even when Leff is less than 2.0 xcexcm, where SCE would normally be a problem, the present transistor experiences minimal sub-threshold currents. The present transistor advantageously employs a net p- or n-type dopant in the polysilicon gate which matches the dopant within the source and drain areas. By utilizing similar dopant within polysilicon as that used in the drain-side LDD and the source/drain region, the present process can be more readily incorporated into existing process flows.
Equally important in the present process is a step used to further reduce HCI. The gate oxide may have barrier atoms incorporated therein by thermally growing the gate oxide in a barrier-containing ambient, a suitable ambient being one that contains nitrogen and oxygen. The result of such a step is a gate oxide containing barrier atoms such as nitrogen. The improvement in hot-carrier reliability is mainly attributed to the presence of nitrogen at the interface between the silicon substrate and the gate oxide (i.e., the Si/SiO2 interface). The presence of nitrogen at the Si/SiO2 interface helps prevent high-energy carriers (electrons or holes) from migrating into the gate oxide since the nitrogen atoms occupy a substantial portion of the migration avenues at the substrate/oxide interface. Further, charge carrier trapping of any electrons that do pass into the gate oxide is limited since strong Sixe2x80x94N bonds exist in place of weaker Sixe2x80x94H and strained Sixe2x80x94O bonds. It is preferred the barrier atoms be nitrogen atoms, however, it is not necessary the barrier atoms be limited exclusively to nitrogen. It is further preferred the barrier atoms exist at the substrate-oxide interface between the drain-side junction and the overlying gate conductor. Barrier atoms thusly placed minimize hot carrier injection into the gate conductor predominantly from the drain-side junction.
In an embodiment of the present invention, a nitrided gate oxide layer is thermally grown across a silicon-based substrate. In order to form such a nitrided oxide layer, the substrate is exposed to a nitrogen and oxygen containing ambient. A layer of polysilicon is then deposited across the oxide layer. Portions of the oxide layer and the polysilicon layer may be removed to form a patterned gate conductor. A lithography step is performed to present a patterned masking layer, i.e., photoresist, across a portion of the gate conductor and the active area on one side of the gate conductor (henceforth referred to as the drain-side junction). Subsequently, the exposed active area opposite the gate conductor side on which the photoresist is patterned (i.e., the source-side junction) may be implanted with either a p-type or n-type dopant, depending on the desired transistor type being formed.
The junctions on opposite sides of the gate conductor may then be implanted using the same type dopant but at a concentration less than the previous implant exclusively in the source region. This implant is therefore a second implant, and is more specifically referred to as LDD implant. The LDD implant, while implanted into both the source and drain regions, net an LDD area only in the drain region since the first implant source regions, being heavily doped, is unaffected by the smaller dopant concentrations of the second implant. Oxide spacers are then formed upon opposed sidewall surfaces of the gate conductor. The spacers are preferably arranged above portions of the heavily doped source-side junction and LDD drain-side junction. The exposed portions of the source-and drain-side junctions are then heavily doped with the same type of dopant previously used. These heavily doped regions result in source and drain areas within the junctions that are aligned to the exposed lateral edges of the oxide spacers.